i've noticed this on my tahoe. i have the instant mpg thing on mine. if i try to slowly cruise up to speed with my heavy *** truck it is like 6mpg during that time and takes for ever to get to speed. but if 1/3 throttle it and the vacuum goes to 0 as it tops the converter the mpg read 4mpg but i'm at 60 in no time.

also when i'm talking about climbing the hills in OD with the tcc locked i'm less than 1/3 throttle and zero vacuum. where as before to hit the same zero vacuum under the same load i would have to be almost full throtle. this is because even though there it no boost registered the turbo is working. its still blowing air into the motor but not enough that the engine can't consume it all with out it being forced. hence boost pressure. so you can run 14.7-13.5 afr with out fear of KR because at that throttle angle and rpm the timing is low. so with lower throttle lower rpm and same afr you have more power because the turbo is blowing air. kinda like maybe a fan in front of a trash bag. it will open the bag but not inflate the bag. there is not enough air pressure to expand the bag. but there is enough airflow to fill up its normal volume with out any work from you ie waving the thing around to open it up.

i hope that helps. probably won't though. lol

 

Things to remember --

Boost is a measure of restriction in the intake tract.

FI adds airflow (and hence increases volumetric efficiency of the engine). At part throttle that VE isn't so out of whack that it requires a ton of fuel (like with higher amounts of boost), and you don't trigger PE mode (if you are a caveman from West Virginia and still use PE on your SC/N2O truck).

While you don't see PSI while driving at part throttle, the turbo is spinning (exclude idle and close-to-no-load driving like deceleration) and increasing airflow into the cylinder making more power. If you watch a vac / boost gauge before and after the turbo install, you will see this for yourself. The turbo spins any time the engine begins to load up whether you see boost on your gauge or not.

You don't see any boost until you are cramming more air into a confined space than exists at atmospheric conditions -- in other words, you are overcoming the engine's ability to draw the ambient pressure into a vacuum.

If you don't drive anywhere without going to full boost, your mileage will suck. If you put monster injectors in (oh say 116 lb/hr lol) so you can run stupid high amounts of boost, your mileage will suck, but it will be better than if you added those injectors to an NA motor capable of making the power.

You will be making better power at part throttle due to all of the above. This translates to less go pedal to accomplish what you want to do, and less PCM freak-out-fuel-dumps (yes, that's a technical tuning term ).

At idle, a drinking straw MIGHT be a restriction on the exhaust side. You don't inhale much air or exhale much air when you are resting quietly and so could probably breath comfortable through that straw, but you do both when you are sprinting and would no doubt pass out, yeah? Same for the engine. There just isn't much airflow or exhaust volume at idle, and those turbo housings don't create any back pressure.

 

I have noticed what your talking about many times when driving around. Not into boost but the vehicle pulls more than a vehicle that is not FI would. Im guessing that this would be right when the engine has 100 percent volumetric efficiency? The point right before you start to have positive manifold pressure...

 

The turbo has to help. Last week I made 900 mile roundtrip going to Nelsons and back home. Truck averaged 16 mpg going 70-75 mph. This is highest hp and mpg out of all my set-ups including NA.

 

Flawed? Not trying to be a dick, but did you spot an error that I missed? I'm merely trying to quantify something in effort to explain it better.

We agree that the turbo is spinning while there is exhaust flowing through the turbine. If the turbo is increasing airflow into the cylinder at a given RPM and load versus the same situation for an N/A engine, then it is essentially providing "boost" to the engine, despite the fact that its still in vacuum. Like going from 12inHg to 10inHg, right? That's an increase in pressure. But, AFR must be maintained and the proportional amount of fuel is provided to complement the increase in cylinder airmass. So you can technically make more power, but that has no bearing on efficiency because it isn't simply consuming more air and less fuel; they're still at the same ratio, and hence no more efficient.

It is acceptable to say that a turbocharged vehicle can cruise and get better mileage, but as many of you have stated, this is because of the tune. Instead of kicking in PE at 55kpa or whatever the stock value is, it comes in later. So not only are you using less fuel, but you're making more torque because of the leaner mixture. The presence of a turbo does not create that effect.

 

i resemble that remark

 

Yes, but it's more accurate to say an increase in airflow, since there is no "boost."

You are almost there... True, if you were hitting the same VE cell, there would be more fuel required, because there is more air in that cell now. HOWEVER, you won't be hitting that cell. Instead, you'll be making the required power in a lower cell now (because there is more airflow in that particular cell at a lower RPM). This is because turbos increase the volumetric efficiency of the engine as I stated above. More airflow = more power at lower RPMs.

That last bolded bit is spurious. You are confusing volumetric efficiency (and it's relationship to torque) with the layman's catchall "efficiency" which is actually a meaningless term. Does a turbo make an engine more efficient? ABSOLUTELY YES. Does it make the vehicle more fuel efficient? This depends on driving style, but the general answer is YES.

You've missed it. It isn't because the tune is set to maximize mileage -- far from it, in fact. Turbo tunes would make a Prius owner cry and demand a lynching. In fact, in many tunes, you tell PE to kick in *earlier* and *faster* than stock. How a turbo saves you mpgs by this is because you **USE** PE mode less.

The reason (as everyone has been saying) that you get better mileage in normal driving is because there is more power "under the curve" (I really hate that term). Because there is more power under the curve, you don't need as much pedal to get to the speed or torque you want to achieve. Less pedal = ??? Yep. Less fuel.

It seems to me that you have a misunderstanding of VE and how the VE of a gas engine relates airflow and power.

 

All i was gonna add was when u complete ur install, drive your truck out of boost, and see if it feels stronger and faster, even tho not in "boost"

I think beserker just finished this thread lol

 

I'm a little late in replying but Erik explained it better than I could have anyways so I'll just quote him again since he did such a good job of it.

I want to throw this video in here of one of my old turbo set-ups. Keep in mind that at idle this motor was around 14in of vacuum. I started recording while on flat surface right before ascending up a notorious incline here in SoCal called the Grapevine. It's a little hard to tell at first but the boost gauge is hovering between 5-7in of vacuum while cruising, once I reach the incline it rises up to about 2-3 psi.

http://www.youtube.com/watch?v=ntd76F264mY


This is simply to illustrate that while cruising and with a slight load, the turbo is creating an increase in airflow into the cylinder as Erik was saying as compared to what a N/A set-up would be doing in the same situation.
Also, listen to the engine and watch the tachometer. At no point does engine speed change. The increase in airflow provided by the turbo isn't a result of the engine getting into PE mode in order to create more exhaust so to spin the turbine faster, it was done by closing down on the wastegate spring to increase turbine RPM. The exhaust flow necessary is already there, it's just being bypassed at the time.


I think your understanding of how much of a restriction these turbo's are is a bit skewed. Remember that these turbo's, once they get going don't cause as much restriction as your implying. True to get them up to speed there's a slight additional load to overcome in both the intake and exhaust track that is not seen in a N/A set-up but once they get going, there is very little restriction.

I'm assuming you've seen these video's before since they are all over youtube, but look how long it takes for this turbo to stop spinning after shutdown.

http://www.youtube.com/watch?v=H1MryAt1dw8

That turbo took 30 seconds to stop spinning and you'll see some guys on tech mention that they've timed their turbo's in the minutes when it comes to coming to a complete stop after shutdown. At WOT on a poorly designed set-up you'll see very high pressures in the hot parts but on just normal everyday driving and cruising around there is very VERY little back pressure caused by turbo's yet a significant increase in airflow into the cylinders by comparison. Newton's laws of motion apply very well here when you consider the mass of the turbo.

 

The efficiency I had in mind was fuel efficiency only, encompassing all others that affect it.

That is exactly what I meant...using PE less.

How would it feel any different than it does now?